Rational design W-doped Co-ZIF-9 based Co₃S₄ composite photocatalyst for efficient visible-light-driven photocatalytic H₂ evolution

Abstract

Herein, a novel W doped Co₃S₄ composite photocatalyst was successfully prepared by MOF (metal organic framework), Co-ZIF-9, as a nanoscaled template via a hydrothermal treatment. Efficient heteroatom doping emerged a large number of joints in Co₃S₄–WS_X, providing multitudinous active sites for hydrogen catalysis. Moreover, due to these unmatched structural and compositional features, the optimized Co₃S₄–WS_X hybrid without introducing any noble metal cocatalysts in the catalytic system also displayed a brilliant performance for hydrogen evolution and prominent stability under long-term visible light irradiation. In particular, the sample of Co₃S₄–WS_X(0.6)), the amount of hydrogen evolution in the five-hour continuous light could reach up to 428.3 μmol, which was increased by 38 times compared with Co-ZIF-9. In addition, some necessary characterization such as XRD, FTIR, SEM, TEM, XPS, BET, UV-vis diffuse spectroscopy, photoelectrochemistry and PL etc. were further examed to support related results and put forward the possible reaction mechanism.

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1. Introduction

As a type of crystalline nanoporous materials, metal–organic frameworks (MOFs) are composed of metal centers and organic linkers. It has attracted much more attention not only owing to their distinct structural topologies and chemical tunabilities but also their multitudinous beneficial characteristics and applications.^1–8 Furthermore, it could be used in catalytic hydrogen production as an effective method to reduce over-reliance on non-renewable fossil fuel combustion, because H₂ as an intermediate medium for storing and transporting energy could be more likely achieved pollution-free and more efficient energy exploitation.^9–15 In the past ten years, a lot of prospective new green energy conversion technologies such as fuel cells, water splitting (OER or ORR), and CO₂RR have been widely probed and developed.^16–21 As a matter of fact, at the design and synthesis of advanced materials for water splitting has been spend great costs in recent years. However, because of the materials that could catalyze these energy conversion reactions in an efficient, durable, environmentally friendly, and low-cost way are still absent, necessary efforts also should be devoted.^22–27

Since meaningful work were further opened up by Yaghi and co-workers, which showed that MOFs are a very promising material in gas sorption, selective separation, catalysis and sensing. The advantage of being exceptionally prominent is that MOFs could be combined with a series of transition metal ions, functionalized imidazolate linkers, and varying topologies. This meaningful design feature brings about a rapid growth of this novel class of crystals in synthetic materials chemistry.^28–30 Among them, the most prominent is 2D metal–organic framework (MOF) nanosheets, and it could achieve the control of structure and functionality over extended length scales by coordination bonding, thus highly ordered metal–organic framework (MOF) nanosheets could be obtained. Additionally, metal–organic framework (MOF) nanosheets are equally important for basic structure–property research and technological development, if a better exfoliation methods could be optimized, and the exfoliation of 3D layered MOFs into their 2D constituents will be more easier.^31–33

Another better strategy for producing functional materials based on MOF is the combination of MOF materials with other materials such as silica, polystyrene, magnetic particles and precious metal particles. In recent years, a new class of materials has been developed that in some ways bridges the gap between MOFs and zeolites, namely, zeolitic imidazolate frameworks (ZIFs). Zeolitic imidazolate frameworks (ZIFs), a family of metal azolate frameworks, composed of tetrahedrally coordinated metal centers interconnected by imidazolate linkers, are a class of MOFs that inherit their structural and physicochemical features from both MOFs and zeolites,^34–38 therefore, it is widely used in many fields. Research has demonstrated that the high surface area and pore structure of ZIFs could constrain the fast diffusion of metal ions during pyrolysis, leading to a large electrochemical surface area and uniform distribution of metal ions with high doping levels, thus further contributing to enhanced ORR and HER electrocatalytic activities.³⁹

Many zeolite topologies have been reported to date by using different imidazolide linkers, such as LTA, POZ and MOZ.^40–42 The study found that Co-ZIF-9 possesses an open-framework structure using a sodalite topology with hexagonal symmetry, it forms a tetrahedral CoN₄ building block by the coordination of cobalt atoms with the nitrogen atom at the 1,3 position on the benzimidazole ester linkage, and this unit is further combined into a Co-ZIF-9.⁴³ It is therefore desirable to find a series of photochemically stable light non-precious metals harvesters to couple with Co-ZIF-9 to construct a stable semiconductor-redox system for promoting the H₂ reduction reaction under visible light. In this work, we selected a novel step ammonia-assisted route to prepare Co-ZIF-9 and Co₃S₄, W element were successfully introduced in situ via hydrothermal treatment. The photocatalytic performance of Co₃S₄–WS_X photocatalysts was obviously better than that of the untreated Co-ZIF-9 photocatalyst, owing to enhanced visible-light absorption and improved charge carrier separation under visible light. In addition, dye-sensitization of the semiconductor as one of the great strategies using visible light, it is necessary to investigate photocatalytic systems that effectively work under visible light region. So we also studied the photocatalytic activity of the catalyst in the dye sensitization system, and the reaction system of the photocatalyst has been optimized and a detailed mechanism for promoting activity was also posed.

2. Experimental section

2.1. Preparation of catalysts

All reagents were of analytical grade and could be used without further purification.

Experimental reagent. Benzimidazole, analytically pure, purchased from Shanghai Macklin Biochemical Co., Ltd; thioacetamide, analytically pure, purchased from Tianjin Damao Chemical Reagent Factory; water soluble EY, purchased from Tianjin Damao Chemical Reagent Factory; sodium tungstate, analytically pure, purchased from Tianjin Damao Chemical Reagent Factory; cobalt nitrate, analytically pure, purchased from Tianjin Kaitong Chemical Reagent Co., Ltd; ammonia water, purchased from Yantai Shuangshuang Chemical Co., Ltd; triethanolamine, analytically pure, purchased from Tianjin Damao Chemical Reagent Factory.

Instrument required for experiment. Vacuum tube furnace (SGL-1400), purchased from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences; electric thermostat blast oven (DHG-9071A), purchased from Shanghai Shenguang Instrument Co., Ltd; electronic analytical balance (FA-1004B), purchased from Shanghai Youke Instrument Co., Ltd; ultra pure water machine (Advance-I-GL), purchased from Chengdu Tangshi Kangning Technology Development Co., Ltd.

2.2. Preparation of Co-ZIF-9 photocatalysts

Benzimidazole (H-PhIM) (0.118 g) and a solid mixture of cobalt nitrate hexahydrate (0.14 g) were dissolved with stirring into 25 mL ultrapure water, then ammonia water (5 mL) was added after 5 min, and then the resulting solution was continued stirring for 30 minutes. After the solution was left to stand for 6 hours, the resulting sample was washed with ultrapure water and filtered, and transferred to a sample catalyst holding tube for storage.

2.3. Preparation of Co₃S₄–WS_X photocatalysts

The prepared Co-ZIF-9 photocatalyst (0.15 g) was added in a beaker, then added 30 mL ultrapure water was dispersed 10 min to form a suspension, 0.329 g sodium tungstate (1 mmol) and 0.15 g thioacetamide were respectively added, after the formation of a homogeneous solution, 9 mL ammonia water was added, continue stirring for 30 minutes. There, the mixed suspension is transferred to a polytetrafluoroethylene reactor for 12 h at 180 °C and naturally cooled to room temperature at the end of the reaction. The precipitate was with deionized water for removed the un-reacted precursor and evaporated to dryness, the obtained products were denoted as Co₃S₄–WS_X(1.0), different amounts were denoted as Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8), respectively.

2.4. Characterization

FT-IR spectra were examined on Thermo Nicolet Avatar 380 FT-IR spectrometer. X-ray diffraction (XRD) crystalline structure analysis using Cu Kα radiation (tube current 40 mA, tube voltage 40 keV). Ultraviolet and visible spectrophotometer (UV-vis) diffuse reflectance spectra was carried out a UV-2550 (Shimadzu) spectrometer. The SEM of all samples were performed by field emission scouldning electron microscopy (JSM-6701F JEOL) at an accelerating voltage of 5.0 kV. High-resolution transmission electron microscopy (TEM) characterization was using an FEI TePCNai TF20 high-resolution transmission electron microscopy with the accelerated voltage of 300 kV. The element composition was detected by X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi). The nitrogen adsorption–desorption isotherms of samples were measured at 77 K with an ASAP 2020M instrument and analyzed by the Brunauer–Emmett–Teller (BET) equation. Photoluminescence data (PL) were obtained by a FLUOROMAX-4 spectrophotometer in a mild environment.

2.5. Photocatalytic hydrogen production tests

Photocatalytic hydrogen production tests were conducted in a quartz reaction flask (62 cm³) which was irradiated by a 5 W white light lamp under magnetic stirring condition (PCX50A Discover 5 W). The specific operation is that 10 mg of photocatalyst powder and 20 mg dye Eosin Y (EY) powder were suspended in 30 mL 15% (v/v) triethanolamine (TEOA) aqueous solution. In order to clear away the air in the reaction flask, the homogeneous solution was purged with bubbling N₂ for 15 min. The amount of hydrogen evolution was measured using gas chromatography (Tianmei GC7900, TCD, 13X column, N₂ as a carrier). All glassware was rigorously cleaned and carefully rinsed with distilled water prior to use.

2.6. Photoelectrochemical measurements

Working electrode preparation. The catalyst powder (5 mg) was added to 250 μL ethanol (containing 200 μL 5% Nafion solution), and the solution was dispersed for 45 minutes to form a homogeneous solution. Then, the suspension of 200 μL was uniformly distributed onto the pretreated FTO with an area of about 1 cm². The paint electrode is dried at room temperature. The electrode was used as the working electrode, the Pt plate as the counter electrode and the saturated calomel electrode as the reference electrode. The 300 Watt xenon lamp is equipped with a filter (over 420 nm) as the lighting source. All PEC measurements were carried out on the three electrode system at the electrochemical workstation (VelSAST4-400, advanced measurement technology, Inc.).

3. Results and discussion

In this study, we used FTIR technology as a diagnostic tool for detecting sample changes. The detail of the test information could be obtained from 2.4. Characterization. Comparison of Co-ZIF-9 and Co₃S₄–WS_X photocatalysts patterns revealed many differences. As you could see from Fig. 1, the bands from 400 cm⁻¹ to −650 cm⁻¹ could correspond to the Co N bonds in Co-ZIF-9. The shape peak at about 745 cm⁻¹ could be assigned to C–H out-of-plane bending vibration,⁴⁴ this could also prove the formation of Co-ZIF-9. But when it is converted to Co₃S₄, the correlation characteristic peak intensity decreased gradually. May be the rupture of C–H, the peak was visibly disappeared with increasing W element usage. The sharp peaks at 1461 cm⁻¹ could be assigned to the ring vibrations, and the absorption at 1606 cm⁻¹ could be assigned to the CN stretching mode, these two points could fully prove the formation of Co-ZIF-9. In addition, the peaks range from 1125 cm⁻¹ to 1140 cm⁻¹ were caused by the vulcouldization of Co-ZIF-9.⁴⁵

Fig. 1 FTIR patterns of Co-ZIF-9 and Co₃S₄–WS_X photocatalysts, (a–f) represent Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8), Co₃S₄–WS_X(1.0), respectively.

The chemical structure of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8) and Co₃S₄–WS_X(1.0) photocatalysts were also compared by XRD techniques. As shown in Fig. 2a, the Co-ZIF-9 pattern shows several distinct peaks. The first peak with a peak of 8.98° could be allocated to the (100) plane of it, and the peaks at 21.5° and 27.5° exhibited two diffraction signals. By comparison with Co₃S₄–WS_X photocatalysts, it was shown that after the addition of S and W elements, the phase of Co-ZIF-9 shown some clear changes. The peaks at 16.102°, 26.667°, 31.361°, 38.049°, 47.279°, 50.225° and 55.006° corresponded to the (111) plane, (220) plane, (311) plane, (400) plane, (422) plane, (511) plane and (440) of linnaeite (PDF#42-1448), it may have been formed during the catalyst synthesis, which agreed with the XPS results.⁴⁶ It is regrettable that the characteristic diffraction peaks corresponding to crystalline WS_X have not been detected. The first reason may be the low amount of W element, another reason may be the high dispersion of WS_X and the relatively low diffraction intensity, it could indicate that the loaded WS_X were amorphous.

Fig. 2 XRD patterns of Co-ZIF-9 and Co₃S₄–WS_X photocatalysts, (a–f) represent Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8) and Co₃S₄–WS_X(1.0) photocatalysts.

In order to study the light absorption capacity of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6) and Co₃S₄–WS_X(1.0) composite photocatalysts, UV-vis spectroscopy characterization was further examed, the relevant results were shown in Fig. 3. The UV-vis absorption spectra of Co-ZIF-9 photocatalysts shown some absorption bands in the UV region, which could be allocated to intra ligand n → π* and π → π* transitions. In addition, due to the spin-allowed d–d transition, and this process could occur at a lower energy level so that Co-ZIF-9 displayed a wide absorption band at 600 nm–650 nm.⁴⁷ The light absorption properties of all modified samples showed a regular change. It was clearly shown that the coupling of WS_X had effect on the intrinsic optical properties of Co-ZIF-9. This result could also prove WS_X evenly presented on the surface of ZIF-9 rather than being doped into it. After the introduction of the W element, the Co₃S₄–WS_X composite catalysts exhibited additional absorption at longer wavelengths, this could be used in part to excite Co₃S₄–WS_X photocatalysts to promote the photocatalytic evolution of H₂.

Fig. 3 (a) The UV-vis diffuse reflection spectra of Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6) and Co₃S₄–WS_X(1.0). (b) The UV-vis diffuse reflection spectra of Co-ZIF-9.

The prepared Co-ZIF-9 and Co₃S₄–WS_X photocatalysts were characterized by SEM and TEM analysis, and the basic structure of the composite catalyst and the important role of W element in enhancing photocatalytic hydrogen evolution were further explored. Fig. 4 a shows the SEM image of the Co-ZIF-9 sheets, it is clear that about 10 μm square sheets are arranged together, some are reunited, and the surface is smooth, the corrugations of some wrinkles in the sample indicate that the resulted Co-ZIF-9 was not a single layer. Fig. 4b shown an SEM image of Co₃S₄–WS_X(0.6) photocatalyst, showing that Co₃S₄–WS_X were successfully prepared. Furthermore, the TEM image of Co₃S₄–WS_X(0.6) (Fig. 4d) also reveals that the Co₃S₄–WS_X evenly distributed. This result indicated that Co-ZIF-9 has successfully transformed into Co₃S₄–WS_X, and the structure has changed little. We could see from Fig. 4e the lattice spacing of 0.50 nm and 0.11 nm can be assigned to the (111) plane and the (440) plane of Co₃S₄, respectively. Tungsten sulfide did not exhibit significant lattice fringes due to its low crystallinity. We could also see from Fig. 4f the elements C, N, O, Co, S, and W were very clearly present in the composite catalyst.

Fig. 4 (a) The SEM pattern of Co-ZIF-9 photocatalysts. (b) The SEM pattern of Co₃S₄–WS_X(0.6) composite photocatalysts. (c) The TEM pattern of Co-ZIF-9 photocatalysts. (d and e) The TEM patterns of Co₃S₄–WS_X(0.6) composite photocatalysts. (f) The EDX spectrum of Co₃S₄–WS_X(0.6) composite photocatalysts.

We used X-ray photoelectron spectroscopy (XPS) to investigate the surface chemical states of Co₃S₄–WS_X(0.6) photocatalysts (Fig. 5). The full scould spectrum corroborates the existence of S, Co and W elements in the sample, with no obvious extra impurities detected, which in agreement with the EDX results. In the S 2p spectra (Fig. 5b), the binding energy of the peaks at 169.0 eV and 170.2 eV could be fitted by SO₄²⁻ species, which maybe originate from the WS_X synthesis and the other two peaks at 162.5 eV and 161.6 eV were attributed to the S 2p_1/2 and S 2p_3/2 orbitals of divalent sulfur ions, respectively.⁴⁸Fig. 5c shown the narrow spectrum of Co 2p contained in the catalyst, we could see that the spectrum was deconvolved into two rotating orbital double peaks and two vibrating satellite peaks. The first doublet at 778.5 eV and 793.6 eV were attributed to Co 2p_3/2 and the second were 781.9 eV and 798.6 eV for Co 2p_1/2.⁴⁹ The narrow spectrum of the W element is not obvious, but this does not affect the relevant results, because EDX could also confirm its existence. The W 4f peaks at 31.3 eV and 31.5 eV could be attributed to the W 4f_7/2 and W 4f_5/2, the binding energy gap between the two is 2.2 eV, which is consistent with the standard W narrow spectrum, and the binding energy at 37.1 eV is attributed to W 5p³.⁵⁰

Fig. 5 X-ray photoelectron spectroscopy (XPS) patterns of Co₃S₄–WS_X(0.6) composite photocatalysts. (a) Survey spectrum, (b) S 2p, (c) Co 2p, (d) W.

As shown in Fig. 6, in order to study the effect of the loading of W on the internal microstructure of Co₃S₄–WS_X(0.6) composite photocatalyst, we tested the specific surface area (SBET), pore size distribution and pore structure of the relevant samples by N₂ adsorption–desorption measurements at 77 K. The nitrogen adsorption–desorption isotherm (Fig. 6a) of Co-ZIF-9 and Co₃S₄–WS_X(0.6) composite photocatalysts are identified as typical type IV with type H₃ hysteresis loop, which is characteristic of mesoporous materials. In addition, an adsorption hysteresis loop appeared in the middle section of this isotherm, which was corresponded to a system in which the porous adsorbent exhibited capillary condensation. However, this H₃ type hysteresis loop did not exhibit adsorption saturation in the higher relative pressure region, which was indicated that the sample to be tested may be composed of flake granular material. As shown in Table 1, the obtained Brunauer–Emmett–Teller (BET) specific surface area (S_BET)of Co-ZIF-9 was only 7.3746 m² g⁻¹, which was smaller than that of Co₃S₄–WS_X(0.2) (7.8235 m² g⁻¹), Co₃S₄–WS_X(0.4) (8.5428 m² g⁻¹), Co₃S₄–WS_X(0.6) (10.9294 m² g⁻¹), Co₃S₄–WS_X(0.8) (8.2354 m² g⁻¹), Co₃S₄–WS_X(1.0) (8.4526 m² g⁻¹). The single-point total volume of pores was 0.025145 cm³ g⁻¹, 0.049453 cm³ g⁻¹, 0.055158 cm³ g⁻¹, 0.064460 cm³ g⁻¹,0.065251 cm³ g⁻¹, and 0.065985 cm³ g⁻¹, respectively. Based on the above discussion, we could draw the following conclusions that improved the specific surface area and grown total pore volume strongly support the fact that the major modify role of W element.

Fig. 6 (a) BET adsorption–desorption isotherms of catalysts, (b) the corresponding pore size distributions.

Table 1 Porous structure parameters of Co-ZIF-9, Co₃S₄–WS_X(0.6) composite photocatalysts

Sample	BET surface area (m^2 g^−1)	Mean pore diameter (nm)	Pore volume (cm^3 g^−1)
Co-ZIF-9	7.3746	6.5506	0.025145
Co3S4–WSX(0.2)	7.8235	7.0152	0.049453
Co3S4–WSX(0.4)	8.5428	8.9256	0.055158
Co3S4–WSX(0.6)	10.9294	13.7756	0.064460
Co3S4–WSX(0.8)	8.2354	13.8256	0.065251
Co3S4–WSX(1.0)	8.4526	13.8298	0.065985

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For the sake of discussing the electron transfer and excited state interactions between EY and Co₃S₄–WS_X composite photocatalysts, the photoluminescence (PL) experiments were further performed. As shown in Fig. 7a, EY solution excited at 480 nm exhibits an extensive emission peak at 540 nm because of its conjugated xanthene structure and strong recombination of photogenerated charges and holes.⁵¹ As Co-ZIF-9 and Co₃S₄–WS_X composite photocatalysts were introduced into the EY solution, different level fluorescence quenching was observed; with the introduce of Co₃S₄–WS_X(0.6) composite photocatalysts this signal progressively dropped to lowest. The results showed that the recombination rate of electron–hole pairs was reduced, ensuring efficient charge transfer from excited EY to Co₃S₄–WS_X(0.6) composite photocatalyst under visible light irradiation.⁵²

Fig. 7 (a) The PL spectra of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(1.0) composite photocatalysts. (b) The time-resolved photoluminescence (TRPL) spectra of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(1.0) composite photocatalysts.

Moreover, the fluorescence emission decays of the EY presence of Co-ZIF-9 and Co₃S₄–WS_X composite photocatalysts are measured, the relevant time-resolved photoluminescence (TRPL) was fitted by a tri-exponential decay model, the equation as follows:

I – the normalized emission intensity; τ_i – a decayed lifetime of luminescence; B_i – the corresponding weight factors.

Fig. 7b shown the results of the fitted, revealing that Co₃S₄–WS_X composite photocatalysts possess more efficient electron–hole pairs separation efficiency compare with Co-ZIF-9; we further calculated the average lifetimes according to eqn (2) is listed in Table 2:

〈τ〉 – the average lifetime, τ_i – a decayed time of the individual components; B_i – the corresponding weight factors.

Table 2 Kinetic analysis of emission decay for EY, EY/Co-ZIF-9, EY/Co₃S₄–WS_X(0.2), EY/Co₃S₄–WS_X(0.6), EY/Co₃S₄–WS_X(1.0) composite photocatalysts

Samples	A 1 (%)	τ 1 (s)	A 2 (%)	τ 2 (s)	A 3 (%)	τ 3 (s)	〈τ〉 (s)	χ ^2
EY	89.73	1.67 × 10^−10	10.27	4.00 × 10^−9	—	—	1.85 × 10^−10	1.572
EY/Co-ZIF-9	10.72	3.73 × 10^−9	26.77	9.79 × 10^−8	62.51	5.51 × 10^−11	8.79 × 10^−11	1.361
EY/Co3S4–WSX(0.2)	11.02	3.77 × 10^−9	23.05	9.81 × 10^−8	65.93	1.26 × 10^−10	1.89 × 10^−10	1.301
EY/Co3S4–WSX(0.6)	0.77	3.83 × 10^−9	2.07	9.90 × 10^−8	97.16	1.02 × 10^−11	1.05 × 10^−11	1.270
EY/Co3S4–WSX(1.0)	9.75	3.59 × 10^−9	12.06	8.02 × 10^−8	78.19	1.51 × 10^−10	1.92 × 10^−10	1.305

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It could be clearly seen from Table 2 that great changes have taken place in the average lifetime of all samples, the average lifetime of EY, EY/Co-ZIF-9, EY/Co₃S₄–WS_X(0.2), EY/Co₃S₄–WS_X(0.6), and EY/Co₃S₄–WS_X(1.0) were 1.85 × 10⁻¹⁰, 8.79 × 10⁻¹¹, 1.89 × 10⁻¹⁰, 1.05 × 10⁻¹¹ and 1.92 × 10⁻¹⁰, respectively. The average life dropped to a minimum when Co₃S₄–WS_X(0.6) composite photocatalyst is introduced, it could be probably attributed to the appropriate cocatalyst addition, which offers a track for photo-generated charge transfer and further improves photocatalytic activity.

We tested the hydrogen production activity of the prepared catalyst in an aqueous solution containing 15% (v/v) triethanolamine (TEOA), related results were shown in Fig. 8a and b. It could be seen from the figure that even after EY sensitization, the hydrogen evolution activity of Co-ZIF-9 was much lower than that of the modified sample Co₃S₄–WS_X. This phenomenon fully highlights the main role of WS_X as an effective carrier for transfer electrons. In particularly, the sample with the best ratio of the load was Co₃S₄–WS_X(0.6) composite photocatalyst, which was shown a higher promoting effect on the photocatalytic hydrogen production process. Specifically, it could be expressed that under the continuous visible light irradiation for 5 hours, the maximum H₂ production amount reached 428.3 μmol, which was achieved a surprising effect. There was a clear contrast that the further increase in WS_X content reduced the evolutionary activity of H₂, which could be explained by the fact that although these samples showed stronger visible light absorption due to doping of WS_X, excessive doping leads to WS_X polymerization. The formation of the body, which could prevent the composite material from absorbing more incident light, thereby reducing the photocatalytic activity.

Fig. 8 (a) Time dependent photocatalytic H₂ evolution over Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8), Co₃S₄–WS_X(1.0). (b) Samples 1–6 represent Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.4), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(0.8), Co₃S₄–WS_X(1.0). (c) Effect of the pH value on the hydrogen production of the Co₃S₄–WS_X(0.6) composite photocatalyst. (d) Long-term photocatalytic test of Co₃S₄–WS_X(0.6) photocatalyst during the repeated runs.

We could conclude that the incorporation of WS_X could effectively improve the photocatalytic activity of Co₃S₄ for H₂ production, and the effect of the pH value of the system on the hydrogen evolution activity needed further to study. As shown in Fig. 8c, due to TEOA as sacrificial agents will be fully protonated under weakly acidic conditions, thereby weakening their ability to supply electrons, a lower hydrogen production activity was exhibited. On the other hand, in this unfavorable reaction system, the lifetime and utilization of the excited state EY molecules were low, thus reducing the photocatalytic activity. In the case where the alkalinity was too strong, the driving force of H⁺ reduction was lowered due to a sharp drop in the amount of H⁺ in the solution, which was suppressed the generation of hydrogen. These results indicated that the EY dye could be efficiently adsorbed on the Co₃S₄–WS_X composite photocatalyst at pH 10 and conducted efficient electron transport, but it was easily quenched by a reduction in TEOA at pH 11.

In addition, we also carried out long-term stability experiments on Co₃S₄–WS_X(0.6) composite photocatalyst. The total time of photocatalytic reaction was controlled at 20 h, and intermittent replacement gas was performed every 5 h, the test results were shown in Fig. 8d. It could be seen from the figure that the photocatalytic H₂ production activity of Co₃S₄–WS_X(0.6) composite photocatalyst remained fairly stable under long-term illumination. The activity of the catalyst decreased slightly after four cycles, which was negligible because the photosensitizer EY had a certain degree of photocorrosion under prolonged illumination.

To further illustrate the H₂ evolution activity of the EY/Co-ZIF-9 and EY/Co₃S₄–WS_X system, the apparent quantum efficiencies (AQEs) for H₂ evolution from EY/Co-ZIF-9 and EY/Co₃S₄–WS_X systems are compared over a wide visible light range of 420–550 nm, as shown in Fig. 9. It could be seen that with the wavelength increasing, AQEs are gradually decreasing. At a wavelength of 420 nm, the EY/Co₃S₄–WS_X system shown the highest AQEs (9.64%), the reason for this phenomenon is that photons at 420 nm have higher energy. Furthermore, another high AQE was obtained at 520 nm, which corresponded to the highest absorption wavelength of EY.

Fig. 9 The apparent quantum efficiencies (AQEs) for H₂ evolution from EY/Co-ZIF-9 and EY/Co₃S₄–WS_X.

We explored the effect of the ratio of the bond between Co₃S₄ and WS_X on charge transfer by means of photoelectrochemical technology. Transient photocurrent response was first examed for the Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6) and Co₃S₄–WS_X(1.0) composite photocatalysts in 0.2 M Na₂SO₄ aqueous solution. The corresponding visible light response could be seen during the switching cycle of the illumination, the Co-ZIF-9 photocatalyst is featureless due to its extremely inefficient charge separation. In contrast, with increasing co-catalyst loading amount a visible increase is showing up, in particular to Co₃S₄–WS_X(0.6) composite photocatalyst. The reason for this could be understood as an excessive amount of WS_X forming aggregates having a larger diameter on the Co-ZIF-9 plate as the amount of the catalyst is increased, so that an effective reactive site was continuously reduced to be detrimental to hydrogen production.

In addition, the linear sweep voltammetry (LSV) of all samples further highlighted the importance of co-catalyst for hydrogen production, as shown in Fig. 10b. The introduction of W had a significouldt effect on the cathode current density during cathodic polarization. The cathodic current densities of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6) and Co₃S₄–WS_X(1.0) composite photocatalysts are constantly increasing, it is clearly demonstrated that loading of a suitable amount of cocatalyst could greatly promote the electron transfer process and further increase the catalyst activity of the photocatalyst.

Fig. 10 (a) Transient photocurrent response for the Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(1.0) composite photocatalysts in 0.2 M Na₂SO₄ aqueous solution under visible light irradiation (λ ≥ 420 nm); (b) LSV curves of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(1.0) composite photocatalysts; (c) EIS Nyquist plots of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6), Co₃S₄–WS_X(1.0) composite photocatalysts.

Last but not least, the kinetics of interface electron transfer could also be obtained by electrochemical impedance spectroscopy. The EIS Nyquist plots of Co-ZIF-9, Co₃S₄–WS_X(0.2), Co₃S₄–WS_X(0.6) and Co₃S₄–WS_X(1.0) composite photocatalysts were shown in Fig. 10c. With the addition of different co-catalysts, ever-reduced semicircle could be observed; when the Co₃S₄–WS_X(0.6) composite photocatalyst is added, the semicircle reached its minimum, indicating that suitable co-catalyst results in a lower charge transport resistance of charges, and a higher separation efficiency of charges and holes, thereby facilitating photocatalytic hydrogen evolution.

Base on the above analysis, the proposed mechanism over Co₃S₄–WS_X was further put forward. As shown in Scheme 1, the possible mechanism proposed was to explain the process of visible light photocatalytic hydrogen production in the EY-sensitized Co₃S₄–WS_X photocatalytic system in an aqueous solution of triethanolamine and the optimization of its related properties. From our perspectives, the Co-ZIF-9 photocatalyst did not provide sufficient electrons for long-term water reduction, because a large portion of those small amounts of excited electrons were prefer to recombine with holes at the limited active sites it provided, leading to a lower H₂ evolution activity. After introducing W on Co₃S₄–WS_X, this cocatalyst captures photogenerated electrons quickly, effectively preventing the recombination of electrons and holes. In addition, since it provided a rich exposed active site, the H⁺ contained in the solution could be largely reduced to produce H₂. Significantly, properly introduced W could maximize their efficiency, which greatly enhanced the photocatalytic H₂ evolution activity of Co₃S₄–WS_X. After using EY as a photosensitizer, it could adsorb to the surface of the obtained composite catalyst, and after the photon was absorbed in EY, the original state is changed to form a singlet excited state EY^1*. However, the EY^1* in the singlet excited state is unstable, and it could reach to the triplet excited state EY^3* with lower energy level through the intersystem transition. When triethanolamine was present, it could reduce the EY^3* that was in the triplet excited state to EY^1*, while it itself was converted to an oxidizing donor for the consumption of holes. Photogenerated electrons could be transferred to Co₃S₄via EY^1* and then transferred to a supported WS_X promoter for water reduction to produce hydrogen.

Scheme 1 The proposed EY-sensitized photocatalytic mechanism in TEOA aqueous solution of hydrogen evolution by water splitting under visible light irradiation(λ ≦ 420 nm).

4. Conclusions

In summary, we have demonstrated that low-crystalline WS_X could be controllably loaded on Co₃S₄ materials through a simple one-step hydrothermal process, and the effective sensitization of organic dye EY to Co₃S₄ composite catalyst makes it an efficient cocatalyst to catalyze the evolution of H₂ under visible light irradiation. Even the Co₃S₄–WS_X(0.2) photocatalyst, which has the lowest W doping level, was increased by up to 14 times, proving that WS_X as co-catalyst could effectively transfer electrons, which could directly or indirectly enhance the light response of the established EY sensitization system to achieve efficient hydrogen production. In addition, supported by some necessary characterization (e.g. PL and electrochemistry), we also proved that nanoparticles could enhance the photocatalytic activity of the catalyst to some extent via the speculated mechanism.

Author contributions

Haiyu Wang conceived and designed the experiments; Zhiliang Jin contributed reagents/materials and analysis tools; Haiyu Wang wrote the paper.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was financially supported by the Chinese National Natural Science Foundation (21862002 and 41663012), the Graduate Student Innovation Project of North Minzu University (YCX18078), The Ningxia Low-grade Resource High Value Utilization and Environmental Chemical Integration Technology Innovation Team Project, North Minzu University and the New Technology and System for Clean Energy Catalytic Production, Major Scientific Project of North Minzu University (ZDZX201803).

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